1. Field of the Invention
The present invention relates to a laser annealing process and a laser annealing system which irradiate a nonmonocrystalline semiconductor film with laser light so as to increase crystallinity of the nonmonocrystalline semiconductor film. In addition, the present invention also relates to a semiconductor film produced by use of the above laser annealing process or laser annealing system. Further, the present invention relates to a semiconductor device and an electro-optic device using the above semiconductor film.
2. Description of the Related Art
Currently, the active-matrix type driving systems are widely used in the electro-optic devices such as the electroluminescence (EL) devices and the liquid crystal (display) devices in which an image is displayed by dot-by-dot driving. In the active-matrix type driving systems, a great number of pixel electrodes arrayed in a matrix are driven through switching devices such as the thin-film transistors (TFTs) arranged in correspondence with the pixel electrodes.
In the active layers of the TFTs, amorphous or polycrystalline semiconductor films (normally silicon films) are widely used. In order to increase the characteristics of the TFT elements, it is desirable that the semiconductor films realizing the active layers exhibit high crystallinity, and it is particularly desirable that the semiconductor films realizing the active layers are monocrystalline.
In the manufacture of the TFTs, typically, an amorphous semiconductor film is formed, and laser annealing of the amorphous semiconductor film is performed for increasing the crystallinity of the film and transforming the semiconductor film into a polycrystalline semiconductor film. In the laser annealing, the semiconductor film is annealed by irradiating the semiconductor film with laser light. However, according to the conventional laser annealing techniques in which the entire surface of an amorphous semiconductor film is uniformly annealed, the increase in the crystallinity is limited, and it is difficult to transform the amorphous semiconductor film into a monocrystal.
In the electroluminescence (EL) devices, the liquid crystal devices, and the like, a great number of TFTs are formed in a predetermined pattern on a single substrate. Therefore, from the viewpoint of the manufacturing efficiency, it is desirable that the crystallinity of the areas in which the TFT elements are to be formed are selectively increased, and it is particularly desirable that the areas in which the TFT elements are to be formed are selectively transformed into monocrystals. However, when a grain boundary exists in an area in which a TFT element is formed, the characteristics of the TFT element deteriorate. Therefore, in order to achieve superior element characteristics and element uniformity, it is desirable that the positions of the areas the crystallinity of which is to be increased can be controlled.
In the technique disclosed in Japanese Unexamined Patent Publication No. 2004-228160, an insulation film having recessed regions is formed, and than an amorphous semiconductor film having a thickness greater than the depth of the recessed regions is formed on the insulation film. Thereafter, the amorphous semiconductor film is transformed into a polycrystalline semiconductor film by heat treatment, and laser annealing of the polycrystalline semiconductor film is performed. In particular, Japanese Unexamined Patent Publication No. 2004-228160 reports that portions of the polycrystalline semiconductor film formed in the recessed regions behave as nuclei of crystal growth, so that the recessed regions and their vicinities can be selectively and substantially completely transformed into monocrystals.
In addition, in the technique disclosed in Japanese Unexamined Patent Publication No. 8(1996)-139331, an amorphous semiconductor film is irradiated with first excimer laser having a first irradiation energy density so as to control the number of generated crystal nuclei, and thereafter the amorphous semiconductor film is further irradiated with second excimer laser having a second irradiation energy density greater than the first irradiation energy density so as to promote crystal growth from the crystal nuclei and transform the amorphous semiconductor film into a polycrystalline semiconductor film.
However, the technique disclosed in Japanese Unexamined Patent Publication No. 2004-228160 includes complicated process steps for forming the insulation film having recessed regions, and is therefore disadvantageous from the viewpoint of the manufacturing cost and the manufacturing efficiency. On the other hand, the technique disclosed in Japanese Unexamined Patent Publication No. 8 (1996)-139331 enables growth of large-size crystals. Nevertheless, this technique cannot control the positions in which the large crystals grow, since, according to this technique, the entire surface of the amorphous semiconductor film is uniformly processed.